The present invention relates to a method for producing solid lignin from a starting material containing lignin.
Lignin has been recognized for a long time as an underexploited renewable resource available for mankind. Today lignin rich streams available at pulp mills are used primarily in heat generation, i.e. they are combusted in recovery boilers. Separation of lignin from pulp mill spent liquor has not become popular mainly due to two reasons: lack of technically feasible solutions for executing the separation and lack of lignin-based value chains. Today, the global trend of developing bio-based applications has made lignin production attractive at a whole new level.
It is known to separate lignin from pulp mill black liquor using the following stages in sequence:
precipitation of lignin by a first stage of the pulp mill black liquor where the pH of the black liquor is lowered by adding a pH lowering agent, preferably CO2
followed by a first dewatering stage while forming a first filter cake,
suspending the first filter cake in a second stage using a second acid or mixture of acids, wherein a lignin suspension is obtained,
dewatering the lignin suspension by a second dewatering stage for forming a second filter cake,
washing the second filter cake by adding a wash liquid to this washing stage, and
dewatering the washed second filter cake obtaining a lignin product, said dewatering being typically made in the last stage of the wash apparatus.
Typically, the above described process is connected to the recovery operations receiving black liquor from a digester of the pulp mill.
These methods are known for example from international publication WO 2006/031175 (corresponds to European patent EP 1794363 and U.S. Pat. No. 8,486,224), and US Patent Application US 2010/0325947 A1. The lignin product obtained by these methods can be used as fuel or raw material for chemicals.
International publication WO 2009/021216 (corresponds to U.S. Pat. No. 8,613,781) presents a method where a moist lignin mass (up to 85% water, normally 45-55%) already separated from black liquor is heated to a “critical temperature” (76-93° C.), which induces a phase transition in the lignin. When the mass is subsequently cooled, the lignin will be separated on the bottom, leaving water as supernatant. According to another embodiment, shown by FIG. 3 of the document, dilute black liquor (not evaporated) is mixed with polymer coagulant agent and with heated diluted phosphoric acid so that the temperature of the mixture remains above 82° C. and the pH is below 3.5, whereafter the mixture is fed to a lignin separation tank, where the lignin is separated upon cooling.
International publication WO2014/116150 presents a method where lignin is first precipitated from black liquor with carbon dioxide, and to the obtained lignin cake, sulphuric acid is added to form an acidic slurry with a pH value in the range 1-3. The acidic slurry is heated to the temperature in the range 100-120° C. and kept in the temperature range a sufficient time so that at least 60% of carbohydrates (hemicelluloses) in the lignin are hydrolyzed, whereafter the slurry is cooled and purified lignin with low carbohydrate content is separated from the cooled slurry by filtration.
The process according to WO2014/116150 produces purified lignin, which after further processing can be used in higher value applications instead of using the lignin only as fuel because of its purity, especially low carbohydrate content. There exists, however, a need to control the physical solid state of the lignin so that the lignin obtained is more easily processable and possibly already in the form which is closer to its form in the final application.